This invention relates generally to photomechanical methods of forming pressure sensitive transfer sheets for reproduction of color images and more particularly, provides a method and means for effecting formation of residue-free color images on sheet substrates for artistic and color proofing purposes.
Many photomechanical processes are known for forming and/or transferring color images to selected substrates by burnishing. Among the common processes are those involving the use of radiation sensitive materials such as photoresists which can be activated by projecting an ultraviolet light on the sensitive surface of a photoresist overlay applied to an underlying ink or color carrying material in turn carried by a substrate. The exposed photoresist located at the imaged areas hardens to become a protective layer bonded to the underlying ink layer. The nonexposed areas are unchanged. The substrate can be bared by exposure to selected solvents in a process which may be referred to as "solvent development". A pressure sensitive adhesive is applied as a thin layer over the imaged and nonimaged areas as a final step in forming the transfer sheet.
The resulting sheet carrying the adhesive coating is placed image side down on a receptor surface and properly registered. The substrate is burnished at the image portions desired to be transferred while same are in contact with the receptor surface.
The substrate is not provided with a release coating, such as a silicone composition, so that when the adhesive is applied over the imaged and nonimaged areas, the adhesive holds to the substrate surface in at the nonimaged areas and will not be released when burnishing takes place during transfer of the imaged portions. Thus one can state that the adhesive has a stronger affinity for the receptor surface at the burnished areas than at the nonburnished areas. The hardened areas defining the image have a stronger affinity for the adhesive than for the substrate and hence adhere to the adhesive at the burnished areas, the underlying ink areas being firmly bonded to the hardened areas (of the imaged photoresist material) and hence transfer with the adhesive and underlying hardened photoresist areas. The bond formed between the nonimaged substrate areas, the respective intermediate layers and the adhesive coated areas is stronger than the bond formed between the receptor surface and said nonimaged adhesively coated areas so that the entire image (which has been burnished) is transferred to the receptor surface without adhesive residue.
One problem encountered in prior aforesaid color imaging methods involve the bleeding of the inks into the body of the underlying surface or into the substrate as stains. Other problems include the formation of undesirable adhesive halos about the transferred images, the transfer of only portions of the color image, the high cost of the specialty inks, the difficulty in mixing the inks with release materials and unsatisfactory bonding to the respective receptor surfaces. Nonuniformity and even pinholes are encountered. Difficulty in transferring fine line images also is experienced with prior methods and image transfer sheet. Registration has been a concern since the image carrier can shift position during burnishing. Additionally, the transferred images often are flat in appearance rather than possessing a desired gloss finish.